In mobile communication networks, there is always a challenge to obtain good performance and capacity for a given communications protocol, its parameters and the physical environment in which the mobile communication network is deployed.
WCDMA (Wideband Code Division Multiple Access) is a radio access technology for packet-switched and circuit-switched services in mobile communication networks. In the uplink, all users in a cell share the same physical wireless channel (band).
FIG. 1 schematically illustrates part of a mobile communications system 1. Typically the mobile communications system 1 is compliant with the WCDMA system and in FIG. 1 those components involved in controlling the transmission power and transmission bit rate of User Equipment UE1 and UE2 in the uplink are illustrated. The network node, in WCDMA denoted Node B, is responsible for controlling the UE transmission power for the control channel, denoted Dedicated Physical Control Channel (DPCCH). The Node B typically strives for achieving a Signal-to-Interference and Noise Ratio (SINR) for the DPCCH that is close to a certain SINR target. The DPCCH contains pilot symbols that are known by the Node B and used by the Node B to estimate SINR in each slot (of length 0.67 ms). If the SINR is below the SINR target, the Node B decides that the UE's transmission power should be increased and if the SINR is above the SINR target the UE's transmission power should be decreased. The Node B communicates its decision by transmitting a single bit in each slot to the UE, the Transmission Power Control (TPC) command, which commands the UE to increase or decrease its transmission power of the DPCCH with a certain step size, e.g. 1 dB. This control loop is denoted Inner Loop Transmission Power Control (ILTPC) in FIG. 1. There is one control loop for each UE in the cell.
Node B is also responsible for scheduling (controlling) the bit rate of uplink data channels for packet-switched services, denoted Enhanced Dedicated Channels (E-DCHs). Let Raise-over-Thermal (RoT) be the total received power at the Node B receiver divided by the thermal noise in the Node B receiver. The total received power includes the interference from all UEs and the thermal noise at the receiver. The scheduler in the Node B typically tries to maximize the total throughput of all E-DCHs in the cell with a side constraint that the RoT should be less than a certain threshold. Yet another typical side constraint is that each UE should be granted a fair throughput compared with other UEs. The main reason for restricting RoT in a cell is to make sure that power limited UEs on the cell edge will be able to communicate with the Node B. RoT is in other words used as a measure for system coverage.
In order to decide the allowed transmission bit rate for each UE, or rather E-DCH, Node B first tries to estimate the RoT contribution for each control channel and data channel for each UE in the latest received frames or slots. A frame is either 2 ms or 10 ms. Furthermore, for each E-DCH, Node B tries to predict what the RoT contribution would be in the upcoming frame(s) for a set of preconfigured bit rates. The scheduler then assigns each E-DCH a bit rate such that the total predicted RoT is below the RoT threshold. Once the bit rates for the E-DCHs have been assigned, Node B transmits scheduling grants to the UEs. The scheduling grants SG (see FIG. 1) are transmitted to the UE on a control channel shared by all UEs in the cell. Scheduling grants are only transmitted when the assigned bit rate for an E-DCH changes. The scheduling grant is sent in the format of a power ratio between the DPCCH and E-DCH. This power ratio corresponds to a modulation and coding scheme and the modulation and coding scheme corresponds to a specific bit rate. If the ILTPC increases the transmitted power of DPCCH with 1 dB, the transmitted power of E-DCH is also increased with 1 dB, unless the UE is power limited. The scheduling grant loop is slow compared with the ILTPC loop.
For a given packet-switched service, the WCDMA system typically tries to achieve a certain Quality of Service (QoS). The QoS is typically measured in block error rate (BLER). For a packed-switched service the optimal BLER is often defined to be the BLER that achieves the maximum throughput (i.e. the achieved information bit rate). The SINR required to achieve a certain BLER for an E-DCH generally depends on a number of different factors. One factor is the transmitted bit rate of the E-DCH. In WCDMA a higher transmission bit rate (typically) requires a higher SINR. The goal of the Outer Loop Transmission Power Control (OLTPC) is to adjust the SINR target of the ILTPC such that a certain BLER or average number of Hybrid Automatic Repeat Request (HARQ) Transmission Attempts (TAs) is achieved.
For E-DCH, the decoding is performed at Node B. Node B sends information to the RNC (radio network controller) about successful and failed transmission attempts (based on CRC, cyclic redundancy check) together with the decoded data. If transmission attempts are successful, the RNC transmits a message to the Node B that instructs the Node B to decrease the SINR target of the ILTPC loop. If one or more transmission attempts failed, the RNC transmits a message to the Node B that instructs the Node B to increase the SINR target of the ILTPC loop. The OLTPC loop is slow compared with the ILTPC loop.
In view of the above there is a need for improved control loops.